Private Branch Exchange (PBX) systems have traditionally connected private enterprise telephony systems to the Public Switched Telephone Network (PSTN) using circuit-switched communications. The PBX systems have evolved to include internet PBX (iPBX) systems that use internet protocol (IP) to carry calls over packet-switched Internet connections. Voice Over Internet Protocol (VOIP) telephony using iPBX systems has become increasingly popular.
The traditional PSTN network provides a very stable network that is typically available at least 99.999% of the time (i.e. less than 5.5 minutes of downtime per year). Although VoIP telephony delivers higher productivity to enterprises over a less reliable IP infra-structure, very few industries are willing to compromise the 99.999% availability offered by the PSTN.
Thus, a fundamental focus in designing iPBX systems is to make them less likely to fail. However, due to the added complexities in PBX and iPBX systems, there is no absolute guarantee that a system would never fail due to internal or external troubles. Therefore, a critical consideration is to consider how to resume service very quickly if it is interrupted. Since diagnosis and reconfiguration can take a considerable amount of time, the fastest response is to move the service to backup iPBX servers.
Usually, a single PBX switch can be integrated with component-redundancy to improve the reliability or fault-tolerance. A typical implementation is to dualize each PBX into an active section and a standby section allowing operations to swap to the standby section when a fault has occurred.
Similarly, redundancy can be introduced at the equipment level. Two co-located PBX switches that are tightly coupled through direct links (e.g., fiber interfaces) can be used to provide real-time standby. The tightly coupled co-located switches typically cannot be used for additional purposes other than as a primary system and a backup system. The use of multiple PBX switches significantly increases the cost of a VOIP telephony system.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.